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// Copyright (C) 2025 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\example userpasses
\ingroup quick3d-examples
\title Qt Quick 3D - User Passes Example
\examplecategory {3D}
\brief Demonstrates creating custom render passes in Qt Quick 3D.
\image userpasses.webp
{A 3D scene rendered using custom user passes}}
The \e {User Passes} example demonstrates how to create custom render passes
in Qt Quick 3D. It implements a deferred lighting rendering pipeline using
multiple user-defined render passes and shows how to handle the complete
rendering sequence: opaque geometry, skybox, 2D items embedded in the scene,
and transparent geometry.
\section1 Disabling internal render passes
By default, Qt Quick 3D uses a set of internal render passes to render the
3D scene. Sometimes you may want to disable these internal passes and
implement your own rendering pipeline using user-defined render passes.
To disable the internal render passes, set the \l {QtQuick3D::View3D::renderOverrides}{renderOverrides}
property of the \l {QtQuick3D::View3D}{View3D} to \c View3D.DisableInternalPasses.
\snippet userpasses/Main.qml disable internal passes
If you disable the internal render passes, you will need to provide the result
of the main color pass for the View3D to be able to display anything on the screen.
\section1 Geometry Buffer Pass
In this example, the first custom render pass is a geometry buffer (G-buffer) pass
that renders the scene geometry into multiple render targets, storing different
material properties in each target. The provided example is a subset of the full
material properties provided by Qt Quick 3D materials, focusing on the properties
needed for a basic deferred lighting implementation.
Our RenderPass is defined in \c GBufferPass.qml:
\snippet userpasses/GBufferPass.qml gbuffer pass
It defines 3 color attachments and 1 depth attachment. The pass requires 3 textures
which are defined as \l {QtQuick3D::RenderPassTexture}{RenderPassTexture} objects inside the RenderPass.
These 3 RenderPassTextures are used as the targets for the color attachments of the pass, and the depth
attachment uses a depth texture provided from outside the pass.
The RenderPass itself is set to \c AugmentMaterial mode, which means that it will augment the materials
of the rendered objects with additional shader code. The augment shader is provided
in the \c gbuffer_augment.glsl file, which outputs the required material properties
to the multiple render targets.
\snippet userpasses/gbuffer_augment.glsl gbuffer augment shader
Here you see how the base color, metalness, worldNormal, roughness and world position are
stored into the 3 color attachments of the G-buffer.
To actually use the G-buffer pass in the rendering pipeline, we need to create an instance
of it in \c Main.qml, and provide the required depth texture:
\snippet userpasses/Main.qml depth texture
\snippet userpasses/Main.qml GBufferPass usage
The G-buffer pass is declared as a child of the main color pass, its consumer.
Nesting a RenderPass inside another RenderPass makes the nested pass render
first, so the lighting stage always samples G-buffers produced in the same frame.
Three \l {QtQuick3D.Helpers::}{RenderOutputProvider} instances are created to provide
references to the rendered G-buffer textures, which will be used in the subsequent lighting pass.
\snippet userpasses/Main.qml gbuffer providers
The \c layerMask property of the G-buffer pass is set to only render objects
that are on ContentLayer.Layer0 and ContentLayer.Layer1. This allows us to control
which objects are rendered in the G-buffer pass by setting their \l {QtQuick3D::Model::layers}{layers}
property accordingly.
\section1 Main Color Pass and SubRenderPasses
Rather than a single flat render pass, this example uses a composite \c mainColorPass
that owns the main color and depth textures and orchestrates all rendering through
a sequence of \l {QtQuick3D::SubRenderPass}{SubRenderPass} children.
Each SubRenderPass shares the same render targets as the outer pass and executes
in order. This makes it straightforward to build a layered rendering pipeline where
each stage adds to the result of the previous one.
\snippet userpasses/Main.qml main color pass
The outer pass sets \c renderableTypes: RenderablesFilter.None so that nothing
renders directly in the parent pass — all rendering is delegated to the
sub-passes. The \c PreserveDepthStencilContents flag ensures that depth values
written by the G-buffer pass are available to every sub-pass.
\section2 Deferred Lighting SubPass
The first sub-pass runs the deferred lighting computation. It renders
the full-screen \c deferredLightingQuad model which samples the G-buffer
textures and evaluates lighting for every pixel.
\snippet userpasses/Main.qml deferred lighting quad
The \c deferredLightingQuad is placed on \c ContentLayer.Layer13 so that it is
invisible to the G-buffer pass and only rendered by this sub-pass.
A \l {QtQuick3D::PipelineStateOverride}{PipelineStateOverride} disables both
depth writes and depth tests for the quad. The G-buffer pass already wrote correct
depth for the scene geometry, so the full-screen quad must not alter or test depth.
\section2 Skybox SubPass
The second sub-pass renders the scene environment skybox using the special
\c RenderPass.SkyboxPass pass mode. Qt Quick 3D takes care of drawing the
skybox geometry using the \l {QtQuick3D::SceneEnvironment::skyMaterial}{skyMaterial}
and \l {QtQuick3D::SceneEnvironment::backgroundMode}{backgroundMode} settings from
the \l {QtQuick3D::SceneEnvironment}{SceneEnvironment}.
A PipelineStateOverride enables depth testing so the skybox is correctly hidden
behind scene geometry, while depth writes are disabled because the skybox sits "at
infinity" and must never occlude geometry.
\section2 Transparent Objects SubPass
Transparent objects cannot be stored in the G-buffer because they require
back-to-front sorted blending. Instead they are rendered last using their
original materials via a dedicated sub-pass.
The transparent cone in the scene is declared as follows:
\snippet userpasses/Main.qml transparent cone
The transparent sub-pass uses a \l {QtQuick3D::RenderablesFilter}{RenderablesFilter}
to select only transparent renderables on the matching layers, and a
PipelineStateOverride enables alpha blending while keeping depth testing active
so transparent objects sort correctly against opaque geometry.
Transparent objects must be rendered \e after all opaque passes so that the
depth buffer is fully populated before blending occurs.
\section2 2D Items in 3D SubPass
Qt Quick 3D can embed standard Qt Quick 2D items inside the 3D scene using
\l {QtQuick3D::Node}{Node} as a container. To include these items in a custom
render pipeline, add a sub-pass with \c passMode: RenderPass.Item2DPass.
\snippet userpasses/Main.qml item2d node
The \c Item2DPass mode instructs Qt Quick 3D to render all Qt Quick items
that are children of 3D nodes into the current render target, composited with
the 3D content already present.
\section1 Rendering to the screen
Finally, to display the result of our custom render passes on the screen,
we need to ensure that the View3D's main color texture is updated with the
result of the main color pass.
\snippet userpasses/Main.qml main texture
The \l {QtQuick3D.Helpers::}{SimpleQuadRenderer} is used to blit the
main color texture produced by the \c mainColorPass to the View3D's framebuffer.
The \l {QtQuick3D.Helpers::}{RenderOutputProvider} exposes the first
color attachment of \c mainColorPass as a texture the SimpleQuadRenderer can
sample.
*/